Unleasing the full potential of Portugal is very much map-dependant. Of course, you can avoid RNG and set up an archipelago map that guarantees you a smooth game, but if you go on more landmass-y maps, you might have to struggle to get your bonuses working.
I tried the Continents and Islands map with Portugal, and I played through the ancient era on multiple maps only to realize that the AI and the city states just LOVE to settle not on the coast, but a tile away from the coast, making them unavailable for trade until they set up a Harbor in the distant future. Of course, all of those maps were perfectly winnable but when I'm playing a civ for the first time, I tend to look for a map that fits their playstyle.
Finally, I found a map in which I had a blue CS on the nearby coast, and I got myself a pretty good start only to realize at T80 that I'm locked in to an inland sea with no canal exits. That was a harsh blow but I intended to keep on until I realized that the civ ability screen is quite misleading: even if the trade route would be between two coastal cities, you can't connect them if the trader would need to touch a single land tile. This means that my core cities could not reach any other target but that one CS. Boom, abandon game.
Nearly cryíng, I rolled a few more games, and alas, the next coastal start turned out to be okay. Although my land movement was blocked by mountains to the south (might be a good thing for defensive purposes), I saw that a single canal city would allow me access for two coastal city states and after I met Japan who had a coastal city in the ancient age (a very rare sight), I knew that this map was the one. And that point, the game was won, I just had to keep sending those juicy trade routes ad nauseam and I snowballed into a peaceful T200 SV on Deity.
So, yeah, Portugal, S tier if you get the right map, but if you play random with no rerolls, you might get a C tier, bonus-less civ experience.